MSOs (multi-service operators) of cable television (CATV) systems provide several services to end users through a fiber optic network, with the final connection to the user often being through a coaxial connection. The services provided by the MSO typically include broadcast analog video and narrow cast digital services, such as data, VoIP, subscription, pay per view and video on demand (VOD) services. The services are generally allocated to a portion of an optical channel, which typically has approximately 1 GHz of available bandwidth. While the bandwidth of a channel is generally constrained by the optical network (the optical network transmitters, optical fiber, channel filter bandwidths and coaxial connection), the number of users connected to the optical network continues to increase, which requires increased demand for bandwidth for the desired services.
In recent years wavelength division multiplexed (WDM) optical transmission systems have been increasingly deployed in optical networks. These include coarse wavelength division multiplexed (CWDM) and dense wavelength division multiplexed (DWDM) systems. Whether a system is considered to be CWDM or DWDM simply depends upon the optical frequency spacing of the channels utilized in the system. Although WDM optical transmission systems have increased the speed and capacity of optical networks, the performance of such systems is limited by various factors such as chromatic dispersion and the fiber nonlinearity, which can cause pulse shape changes in the case of baseband digital signals and distortions in case of analog signals. These impairments degrade the quality of the optically transmitted information. Fiber nonlinearities, for example, can give rise to the generation of undesirable optical signals whose wavelengths fall within the filter passbands of the various channels within the system. If the power levels of these undesirable signals are sufficiently large they will corrupt the information being transmitted by the optical signals that are impacted by these impairments.
One common nonlinear impairment in an optical fiber communication system having multiple wavelengths is four wave mixing (FWM). The FWM process is also known by other names such as parametric interactions. Parametric interactions, or FWM, occur when the photons comprising the desired optical signals of a system elastically scatter in a silica fiber (and other materials). The material comprising the optical fiber acts as catalyst for elastic scattering. During the course of elastic scattering events incident photons interact with the propagation medium (the optical fiber material) through collisions with the material. The scattering event is elastic because none of the optical energy transported by the incident photons is lost to the fiber itself. This is unlike Raman scattering, which is an inelastic scattering process with some of the optical energy being lost to the transmission medium. Instead, the incident photons' energy is instantaneously converted back into optical energy. The scattered photons' energy may or may not be at the original frequencies of the incident photons that initiated the elastic scattering event. Both energy and momentum are conserved during the elastic scattering event:ω1+ω2=ω3+ω4 and {right arrow over (k)}1+{right arrow over (k)}2={right arrow over (k)}3+{right arrow over (k)}4,where ωi is the frequency of the ith signal and ki is its associated wavevector, is Planck's constant divided by 2π. As a result of the scattering event two photons are annihilated and two other photons are created. If the energy relationship is rearranged it becomes clear how three co-propagating waves interact to create a fourth signal: ω1+ω2−ω3=ω4. The fourth signal at ω4 is a new signal that was not originally present in the system at the transmitter launch site. The signal at ω4 is created as a result of the inelastic scattering events (four wave mixing processes) occurring within the optical fiber. The fourth signal at ω4 is the undesirable nonlinear impairment. If ω4 is too close in frequency to one of the original signals (i.e. one of the DWDM channels) it can potentially interfere with and corrupt the information being transmitted by the desired signal in its vicinity. The consequence of momentum conservation is that the four wave mixing terms are largest under conditions when the dispersion is smallest amongst the interacting optical signals. This situation arises when the optical DWDM signals are located near the zero dispersion point of the optical fiber (i.e. in the vicinity of 1311 nm for ordinary single mode optical fiber) or when the signals are closely spaced together (i.e. when the signals are spaced at distances of 100 GHz or less). In the case of a near zero dispersion system the optical pump and signal waves are propagating at nearly identical group velocities through the media. The zero dispersion wavelength of a transmission media refers to the wavelength at which an optical signal will have no change in (inverse) group velocity with respect to changes in its optical frequency. The zero dispersion wavelength differs for different transmission media. The dispersion will generally increase as the wavelength difference between the optical signal and the zero dispersion wavelength increases. However if the signals are closely spaced in frequency the difference will not be great enough to disrupt the conservation of momentum requirement to any great degree.
As noted above, four-wave mixing is a nonlinear process in which the interaction of three fields leads to the generation of a fourth field. That is, FWM is the induced combination of three wavelengths to produce one or more new wavelengths. Two of the three combining wavelengths can be degenerate such that two wavelengths are combined to produce one or more new wavelengths. Optical power is removed from the combining wavelengths and transferred to the new wavelengths in the FWM process. FWM is especially problematic in optical communications if the new wavelengths produced by the FWM process overlap the assigned wavelengths of existing DWDM channels because it is difficult to distinguish between the legitimate optical data signals at these existing WDM channels and the error signal superimposed thereon as the result of FWM. Therefore, it is important to suppress FWM.
FIG. 1 illustrates the four-wave mixing process. If three signals at frequencies f1, f2, and f3 propagate through a single mode fiber, light at a frequency f4=f1+f2−f3 will be generated. If any two of the frequencies are identical the FWM process is referred to partially degenerate and if all three frequencies are identical the FWM process is referred to as totally degenerate. More generally, in a system with N channels, three signals at frequencies fi, fj and fk produce signals at frequencies fi±fj±fk, where i, j and k vary from 1 to N. This results in N2(N−1)/2 interfering signals. FIG. 2 shows an example with three channels, which produce 12 interfering terms.
Optical WDM networks typically allocate a portion of the spectrum about a center frequency of the nominal channel wavelength for signal transmission. Two different optical wavebands or windows are commonly utilized, one centered around a wavelength of about 1310 nm and another centered around a wavelength of about 1550 nm. Signals operating in the 1310 nm window typically occupy wavelengths between about 1280 nm and 1330 nm, whereas signals operating in the 1550 nm window typically occupy wavelengths between about 1530 and 1565 nm. In dense wavelength division multiplexing (DWDM) systems, channel spacings of less than 1 nm are typically used with wavelength bands centering usually centered around 1550 nm. In the 1550 nm window, the International Telecommunication Union (ITU) specifies a DWDM standard using 100 GHz or 200 GHz spacing between signal wavelengths, which is equivalent to 0.8 nm and 0.4 nm, respectively.
The widely used optical wavelengths in a typical CATV application, e.g. those in the 1310 nm window, exhibit little relative dispersion between adjacent ITU frequencies, and hence are particularly affected by the FWM effect when used in a DWDM system. For this reason CATV MSOs have been generally prevented from using a DWDM approach to increase bandwidth because of the degradation of the signal quality from FWM interactions. As a result, in order to meet the increased demand for additional bandwidth, CATV MSOs may be required to install more optical fiber to carry additional channels, and then segment their subscriber base between the newly installed optical fiber and the existing fiber. However, this approach requires a significant capital investment for the MSOs and often involves the negotiation of additional access rights to install the optical fiber. Alternatively, the CATV MSOs may use other wavelengths which are less affected by FWM such as those in the 1550 nm window. However use of these wavelengths require more expensive optical components, e.g. lasers and nodes, significant changes to their existing optical network and significant capital investment as well. Hence it remains desirable to operate DWDM systems in the 1310 nm window for a number of reasons. This is primarily because systems operating in the 1310 nm window are generally simpler and less expensive to implement than systems operating in the 1550 nm window. For example, laser transmitters operating in the 1310 nm window are significantly less expensive than lasers operating in the 1550 nm window. Moreover, because of the higher dispersion in the 1510 nm window, additional components such as dispersion compensators are often needed when operating in the 1550 nm window.
Accordingly, it is desirable to reduce the levels of FWM distortions that arise among the individual channels in a DWDM optical system. This is particularly true in the case of a system utilizing optical channels that are located near the zero dispersion wavelength of the transmission medium.